1. Field of the Invention
The present invention relates to a high-frequency switch for changing signal paths in a high-frequency circuit, for example in a portable telephone, and more particularly to a high-frequency switch having four ports and utilizing diodes.
2. Background Art
In a portable telephone, in some cases, two antennas, or one antenna and one external terminal, are commonly used in a transmitter and a receiver. A conventional switch circuit having such a construction is illustrated in FIG. 7.
A switch circuit 151 has a construction wherein 3-port switches 152 and 153 are connected to each other. The switch 152 has first, second and third ports P21, P22 and P23. Similarly, the switch 153 has first to third ports P31, P32 and P33. An antenna ANT is connected to the second port P22 of the switch 152 and the third port P23 is used as an external connection terminal EXT. A second antenna is sometimes connected to the third port P23. That is, in a portable telephone for use in a vehicle or the like, a second antenna which is mounted on the vehicle can also be connected. As another example of use of the external connection terminal EXT, there are cases where a prescribed signal is input through the external terminal when measuring electrical characteristics of the receiver in a portable telephone containing the switch circuit 151 therein.
In the switch 152, the first port P21 can be changed over to the second port P22 or to the third port P23. The first port P21 is connected to a first port P31 of the switch 153.
The first port P31 of the switch 153 can be changed over between a second port P32 and a third port P33. The second port P32 is connected to the transmitter Tx and the third port P33 is connected to the receiver Rx.
By using the above-mentioned switch 151, either the antenna ANT or the external terminal EXT can be connected to either the transmitter Tx or the receiver Rx.
FIG. 8 shows a known type of high frequency switch using diodes which can constitute the 3-port switches 152 and 153. A high frequency switch 161 has first to third ports P61 to P63 corresponding to either the first to third ports P21 to P23 or to the first to third ports P31 to P33. The port P61 is connected through a capacitor 164 to a cathode of a diode 165a. One end of a distributed constant line 167a is electrically connected to a point A of connection between an anode of the diode 165a and a capacitor 166a. Assuming that the wavelength of a high frequency signal flowing through this switch 161 is xcex, the distributed constant line 167a is composed of a transmission line having a length of xcex/4 or less. The other end of the distributed constant line 167a is connected through a capacitor 168a to ground potential. One end of a resistor 169a is connected to a point of connection between the distributed constant line 167a and a capacitor 168a and the other end thereof is connected to a control voltage terminal Vc1.
Also, the first port P61 is connected through the capacitor 164 to a distributed constant line 171 constructed the same way as the distributed constant line 167a. The other end of the distributed constant line 171 is connected to ground potential.
Further, the cathode of a diode 165b is connected through the capacitor 164 to the first port P61. The anode of the diode 165b is connected through a capacitor 166b to the third port P63. Also, as with the diode 165a, a series circuit consisting of a distributed constant line 167b and a capacitor 168b is connected between the anode side of the diode 165b and ground potential. One end of a resistor 169b is connected to a point of connection between the distributed constant line 167b and the capacitor 168b and the other end thereof is connected to a control voltage terminal Vc2.
In the high frequency switch 161, by applying different control voltages to the control voltage terminal Vc1 and the second control voltage terminal Vc2, it is possible to realize a state of connection wherein the port P61 is connected to the second port P62 or wherein the port P61 is connected to the third port P63. For example, when a positive control voltage is applied to the control voltage terminal Vc1 and, on the other hand, a negative control voltage is applied to the control voltage terminal Vc2, a forward bias voltage is applied to the diode 165a and a reverse bias voltage is applied to the diode 165b. That is, since the capacitors 166a, 168a, 164, 166b and 168b block the flow of direct current, they block the control current supplied from the control voltage terminal Vc1 which is thereby caused to flow through a circuit portion including the distributed constant line 167a, the diode 165a and the distributed constant line 171. As a result, the diode 165a goes on. On the other hand, a reverse bias voltage is applied to the diode 165b, with the result that the diode 165b goes off.
Also, in order to guide the flow of a high frequency signal supplied from the second port P62, since the distributed constant line 167a is constructed as mentioned above, one end of the distributed constant line 167a can be set to have ground potential with respect to high frequencies by providing the capacitor 168a with a large capacitance. As a result, the impedance of a series circuit composed of the distributed constant line 167a and the capacitor 168a as viewed from the connection point A becomes infinite with respect to high frequencies by impedance inversion. Accordingly, a high frequency signal supplied from the second port P62 is caused to flow into the first port P61.
On the other hand, when a negative control voltage is applied to the first control voltage terminal Vc1 and a positive control voltage is applied to the second control voltage terminal Vc2, conversely to the above, a reverse bias voltage is applied to the diode 165a and a forward bias voltage is applied to the diode 165b. Accordingly, the diode 165a goes off and the diode 165b goes on. As a result, no signal flows between the second and first ports P62 and P61 and a signal flows between the first and third ports P61 and P63. In this case as well, since the impedance of a series circuit composed of the distributed constant line 167b and the capacitor 168b becomes infinite with respect to high frequencies, no high frequency signal flows in the distributed constant line 167b. 
The distributed constant lines 167a and 167b respectively form the current paths for causing the control currents to flow through the diodes 165a and 165b and perform the function of increasing with respect to high frequency signals the impedances of the distributed constant lines 167a and 167b sides as viewed from the connection points A and B and thereby decreasing the insertion loss and reflection loss.
As mentioned above, in the high frequency switch 161, by applying positive and negative control voltages to the control voltage terminals Vc1 and Vc2, it is possible to terminate the connection between the first port P61 and the second port P62 and establish the connection between the first port P61 and the third port P63, and vice versa.
The switch circuit 151 illustrated in FIG. 7 is constructed using a pair of the high frequency switches 161 as the switches 152 and 153. That is, in the switch circuit 151, two 3-port high frequency switches are used and the first ports thereof are connected to each other.
As mentioned above, since the switch circuit 151 is constructed by connecting the two high frequency switches 152 and 153, a high frequency signal passes through the two switches. For example, a transmission output supplied from the transmitter Tx passes through the two switches 153 and 152 before arriving at the antenna ANT. Similarly, a high frequency signal input from the antenna ANT passes through the switches 152 and 153 before being supplied to the receiver Rx. For this reason, it is unavoidable that the insertion loss becomes large and it has been strongly demanded that this insertion loss be reduced. Another problem is that because of the unavoidable increase in insertion loss, it is necessary to increase the transmission output when transmitting, while, on the other hand, a decrease in the gain results when receiving.
In the above-mentioned switch circuit 151, since the switches 152 and 153 are each constructed using the high frequency switches 161, it is necessary in each of the switches 152 and 153 to apply control voltages to the first and second control voltage terminals and accordingly two power sources for supplying the control voltages have been needed in each of the switches 152 and 153. As a result, complex wiring patterns for the power sources have had to be formed on the circuit board.
An object of the present invention is to provide a high frequency switch in which the insertion loss is small, the number of the constituent parts can be reduced, and the wiring patterns for supplying the control voltages can be simplified.
In order to achieve the above object, according to a first aspect of the present invention, there is provided a high frequency switch having first to third ports and enabling changeover between a state of connection between the first and second ports and a state of connection between the first and third ports, comprising a first diode connected between the first and second ports; a second diode connected between the first and third ports, the first and second diodes being connected to the first port in the same direction with respect thereto, coupling capacitors connected between the first port and the first and second diodes, between the second port and the first diode, and between the third port and the second diode; distributed constant lines and capacitors connected in series with each other between the coupling capacitors connected to the first to third ports and reference potentials, and control voltage terminals connected to points of connection between the distributed constant lines and the capacitors; wherein a voltage having a fixed value is applied to the control voltage terminal connected to the point of connection between the distributed constant line and the capacitor connected to the first port.
According to a second aspect of the present invention, there is provided a high frequency switch having first to fourth ports and enabling changeover of each of the first and second ports between the third and fourth ports, comprising first to fourth diodes connected between the first and third ports, between the first and fourth ports, between the second and third ports and between the second and fourth ports, the first and second diodes being connected to the first port in the same direction with respect thereto and the third and fourth diodes being connected to the second port in the same direction with respect thereto, distributed constant lines and capacitors connected in series with each other between the third and fourth port sides of the first to fourth diodes and a reference potential, control voltage terminals connected to points of connection between the distributed constant lines and the capacitors, a pair of third distributed constant lines respectively connected between the first and second ports and the reference potential, a capacitor connected between the pair of third distributed constant lines and the reference potential, and a fixed voltage terminal connected to a point of connection between the third distributed constant lines and the capacitor and for having applied thereto a control voltage having a fixed value.
According to a third aspect of the present invention, there is provided a high frequency switch having first to fourth ports and enabling changeover of each of the first and second ports between the third and fourth ports, which comprises first to fourth diodes connected between the first and third ports, between the first and fourth ports, between the second and third ports, and between the second and fourth ports, the first and third diodes being connected to the third port in opposite directions and the second and fourth diodes being connected to the fourth port in opposite directions, distributed constant lines and capacitors connected in the series with each other between the first to fourth ports and reference potentials, fixed voltage terminals connected respectively to points of connection between the distributed constant lines and the capacitors in the series circuits composed of the distributed constant lines and capacitors connected to the first and second ports, for applying fixed voltages to the points of connection, and voltage control terminals connected to points of connection between the distributed constant lines and the capacitors in the series circuits composed of the distributed constant lines and capacitors connected to the third and fourth ports.
In the first aspect of the invention, a positive power source voltage Vcc, for example, can be applied to the control voltage terminal connected to the point of connection between the distributed constant line and the capacitor both connected to the second port, the control voltage terminal connected to the point of connection between the distributed constant line and the capacitor connected to the third port being grounded, and the control voltage terminal connected to the point of connection between the distributed constant line and capacitor connected to the first port being grounded. As a result, the first diode can be turned xe2x80x9conxe2x80x9d and the second diode can be turned xe2x80x9coffxe2x80x9d. That is, a signal can be caused to flow between the first and second ports by using only one type of power source voltage Vcc.
Conversely, a positive power source voltage Vcc, for example, can be applied to the control voltage terminal connected to the point of connection between the distributed constant line and capacitor connected to the third port, and each of the remaining control voltage terminals can be grounded. As a result, the first diode goes off and the second diode goes on, whereby it is possible to cause a signal to flow between the first and third ports.
When causing a signal to flow between the first and second ports or between the first and third ports, the impedance of a series circuit composed of the distributed constant line and the capacitor, as viewed from the end thereof which is opposite to the reference potential, becomes infinite with respect to high frequencies, so that a high frequency signal does not, flow toward the reference potential. That is, this signal flows between the first and second ports or between the first and third ports as mentioned above.
Also, as apparent from embodiments described later, a voltage having a constant value which is applied to the control voltage terminal connected to the point of connection between the distributed constant line and the capacitor both connected to the first port need not be a positive power source voltage Vcc and may be a negative control voltage xe2x88x92Vcc, or may be grounded. This constant voltage is made equal to the control voltage applied to one of the remaining two control voltage terminals and defined as a second voltage, with the control voltage applied to the remaining one control voltage terminal being defined as a first voltage. In this case, since one of the first and second voltages can be set to have ground potential, according to the first aspect of the invention it is possible to realize a state of connection between the first and second ports or between the first and third ports by using only one control voltage Vcc or xe2x88x92Vcc.
Accordingly, it is possible to simplify the wiring patterns for supplying a power source voltage necessary for the high frequency switch and also to facilitate the design of a printed circuit board on which the high frequency switch is mounted. Accordingly, it is possible to promote miniaturization of an electronic device such as a portable telephone having the high frequency switch incorporated therein.
In the second aspect of the invention, by turning the first diode xe2x80x9conxe2x80x9d, a signal flows between the first and third ports and, in this case, the remaining second to fourth diodes are turned xe2x80x9coffxe2x80x9d. Similarly, by turning the second diode xe2x80x9conxe2x80x9d, a signal flows between the first and fourth ports and, in this case, the remaining diodes are turned xe2x80x9coffxe2x80x9d. Further, by turning the third diode xe2x80x9conxe2x80x9d, a signal flows between the second and third ports and, in this case, the remaining diodes are turned xe2x80x9coffxe2x80x9d. Similarly, by turning the fourth diode xe2x80x9conxe2x80x9d, a signal flows between the second and fourth ports and the remaining diodes are turned xe2x80x9coffxe2x80x9d.
That is, by applying the control voltages from the control voltage terminals so as to turn any one of the first to fourth diodes xe2x80x9conxe2x80x9d as mentioned above, it is possible to realize a state wherein either the first or second port is connected to either the third or fourth port. In this case, as in the case of the first invention, a first voltage is applied as the control voltage to the control voltage terminal connected to the point of connection in the series circuit composed of the distributed constant line and the capacitor and connected to the diode desired to be turned xe2x80x9conxe2x80x9d so that this diode goes on, and a second voltage different from the first control voltage is applied to each of the remaining control voltage terminals and the fixed voltage terminal. Furthermore, since one of the first and second voltages can be made to have ground potential, it is sufficient that only one control voltage be provided as in the case of the first aspect of the invention. Accordingly, it is possible to simplify the wiring patterns for supplying the power source voltage and therefore facilitate the design of the wiring patterns for supplying the power source voltage on a printed circuit board or the like, thereby enabling miniaturization of an electronic device as in the case of the first invention.
In addition, whereas when the switch circuit 151 is constructed using the conventional high frequency switches 152 and 153, a high frequency signal passes through two diodes, in the second aspect of the invention a high frequency signal passes through only one diode. Accordingly, the insertion loss can be reduced to xc2xd that in the case where the conventional switch circuit 151 is used, and the service life of the elements constituting the high frequency switch can also be increased. Thus, for example, when used in a portable telephone, the second aspect of the invention has the effect of reducing the consumption of batteries and enabling long-duration communication by telephone.
In the third aspect of the invention, by turning any two of the first to fourth diodes xe2x80x9conxe2x80x9d and turning the remaining diodes xe2x80x9coffxe2x80x9d, it is possible to realize a state of connection wherein either the first and third ports or the first and fourth ports are connected to either the second and third ports or the second and fourth ports. In the third aspect of the invention, when the high frequency switch thereof is used, a first voltage is applied to one of the pair of fixed voltage terminals respectively connected to the first and second ports and a second voltage is applied to the other thereof. One of the first and second voltages may be ground potential. Accordingly, each of the above-mentioned states of connection can be realized using only one control voltage. Accordingly, as in the case of the first and second aspects of the invention, since the wiring patterns for supplying the power source voltage can be simplified, it is possible to simplify the wiring patterns on a printed circuit board on which the high frequency switch is mounted, thereby enabling promotion of miniaturization of an electronic device such as a portable telephone in which the high frequency switch is incorporated.
Also, in an xe2x80x9conxe2x80x9d state, a high frequency signal passes through only one diode as in the second aspect of the invention. Accordingly, the insertion loss can be reduced to xc2xd compared to the conventional switch circuit 151 using the 3-port high frequency switches 152 and 153. Further, whereas in the conventional switch circuit 151 six transmission lines were needed, in the third aspect of the invention only four transmission lines are needed. Accordingly, from this viewpoint as well, it is possible to achieve miniaturization of an electronic device such as a portable telephone in which the high frequency switch is incorporated.
A high frequency switch according to one of the above-mentioned first to third aspects of the present invention preferably further comprises a distributed constant line and a capacitor connected in series with each other and connected in parallel with at least one of the diodes. In a circuit construction wherein a distributed constant line and a capacitor connected in series are connected in parallel with at least one diode, a parallel resonance circuit is constructed by the capacitance and the impedance of the distributed constant line when this diode is in an xe2x80x9coffxe2x80x9d state. Accordingly, by making the resonance frequency of this parallel resonance circuit the same as the frequency of the high frequency signal flowing through the switch, it is possible to increase the impedance when the diode is in an xe2x80x9coffxe2x80x9d state, thereby enabling enhancement of the isolation characteristic. Note that in this case the capacitor functions to prevent bypass of a direct current to a circuit portion including the distributed constant line connected in parallel with the diode.
Preferably, at least one capacitor is connected between an end of at least one of the diodes and the reference potential. By connecting at least one capacitor between an end of at least one diode and the above-mentioned reference potential, it is possible to adjust the characteristic impedance by this capacitor, thereby enabling a decrease in the insertion loss and the reflection loss. Further, it is possible to shorten the length of the above-mentioned first distributed constant line, thereby contributing to miniaturizing the high frequency switch.
Preferably, a resistor is connected in parallel with at least one of the diodes. In a construction wherein a second resistor is connected in parallel with at least one diode, electric charge accumulated in the capacitance occurring when this diode is in an xe2x80x9coffxe2x80x9d state is discharged to this resistor simultaneously with changeover thereof to an xe2x80x9conxe2x80x9d state. As a result, it is possible to smoothly perform a switching operation from the xe2x80x9coffxe2x80x9d state to the xe2x80x9conxe2x80x9d state of the diode.
Other objects, features and advantages of the present invention will become apparent from the following description taken in conjunction with the accompanying drawings.